I'm having the same problem with Cisco IPSEC-style VPN connections. They're very unreliable with Big Sur beta, with frequent disconnections as well as problems establishing SSL and SSH connections, even with the VPN up. Non-encrypted connections work better (e.g. HTTP) than encrypted (e.g. HTTPS).
I tried the VPN Tracker 365 beta and it was no better. Still unreliable, so presumably it relies on the same underlying network stack. I have reported using the Feedback Assistant, and I recommend anyone else with this problem should do the same.